Bi-level and multi-level devices have limited tonal range. Intermediate tones, such as varying shades of gray, must be represented by halftones. Halftoning is a process by which continuous-tone colors are approximated by a pattern of pixels that can achieve only a limited number of discrete colors. The most familiar case of this is the rendering of gray tones with black and white pixels, as in a newspaper photograph.
There are many conventional techniques for halftoning. Dithering, stochastic screens, and error diffusion are all different types of halftoning techniques. When a particular halftoning technique is used, the resulting image is comprised of halftone patterns. In other words, halftone patterns are what the halftoning techniques create.
Contemporary halftoning techniques have parameters specific to each technique. For example, dithering techniques include order and unordered dithering. Error diffusion is an example of unordered dithering. Ordered dithering is usually implemented using a threshold array. Furthermore, ordered dithering can be further sub-categorized into clustered or dispersed dot dithers.
With clustered dot dithering, the arrangement of the gray levels tends to result in the formation of clumps or clusters. With dispersed dot dithering, the successive gray threshold values in the array are spread or dispersed away from each other as much as possible. Stochastic screens are a class of dispersed dot dithering in which the appearance of the halftoned result is similar to that of an unordered dither. Stochastic screens are preferred over unordered dithering in that it has much better computational efficiencies (both in time and in memory usage).
When an image is halftoned, the parameter or parameters (i.e., ordered, threshold array) specific to the halftoning technique remains constant during the halftoning process. For example, the same array is used each time the image is halftoned.
For display devices, such as computer monitors and televisions, images are presented as frames that are refreshed or repeated many times a second. This refreshing or repeating of the image occurs regardless of whether the image itself is changing over time. So, for a picture displayed on a computer monitor, a movie played on television, or a movie played in a theater, each frame is refreshed or repeated many times a second.
However, as discussed earlier, the parameters in contemporary halftoning techniques remain constant while each frame is repeated. This can create artifacts in the image, in that the halftone patterns become visible to the human eye. This is especially true for animated images, because the halftone pattern remains fixed over time while the frames in the image move and change over time.